An arrayed waveguide grating (AWG) is a device known as a multiplexing/demultiplexing circuit of a wavelength division multiplexing communication apparatus. Similarly, a dispersion compensation circuit having a lens structure in the arrayed waveguide grating is also a known device. Generally, the performance of an optical element depends on its size and precision. If the optical path length of each waveguide has no error, the frequency resolution of an arrayed waveguide grating is substantially determined by a difference in the optical path length between the longest and shortest waveguides of arrayed waveguides. However, when the optical path length becomes long, an error in the optical path length is inevitable. On the other hand, the upper limit of the optical path length is fixed by the size of a substrate. To solve these problems, a conventional technique has followed the procedures as shown below: first, (a) a circuit is reduced in size by enlarging the relative refractive index difference between waveguides, and a layout in which the optical path length is as long as possible is designed within the range of a given substrate size, or alternatively, the optical path length is increased by enlarging the substrate size, thereafter (b) a phase error (optical-path-length error) in a delay waveguide part is measured through interference spectrometry by using low-coherence broadband interferometry or by providing a interference light circuit on the same substrate; and subsequently (c) the phase error is compensated by changing the refractive index by such means as irradiating the arrayed waveguides with an ultraviolet laser beam.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-20837
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2005-250022